pH

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N-UNCOUNT: also a N, oft N of n, N num

The pH of a solution indicates how acid or alkaline the solution is. A pH of less than 7 indicates that it is an acid, and a pH of more than 7 indicates that it is an alkali.

...the pH of sea water...

Skin is naturally slightly acidic and has a pH of 5.5...

The fluid that emerges from the vents is acidic (pH 3) and hot.